Helium effects on void formation in 9Cr-1MoVNb and 12Cr-1MoVW irradiated in HFIR

Up to 2 wt % Ni was added to 9Cr-1MoVNb and 12Cr-1MoVW ferritic steels to increase helium production by transmutation during HFIR irradiation. The various steels were irradiated to approx.39 dpa. Voids were found in all the undoped and nickel-doped steels irradiated at 4000C, most of them at 5000C, but not in any of them at 300 or 6000C. Bubble formation, however, was increased at all temperatures in the nickel-doped steels. Maximum void formation was found at 4000C, but swelling remained less than 0.5% even with up to 440 appM He. Irradiation at 300 to 5000C caused dissolution of as-tempered M23C6 precipitates and coarsening of the lath/subgrain structure in the 9-Cr steels, whereas the microstructure generally remained stable in the 12-Cr steels. Irradiation in this temperature range also causd compositional changes in the as-tempered MC phase in all the steels, and produced combinations of fine M6C, G, and M2X precipitates in various steels. The subgrain boundaries appear to be strong sinks that enhance resistance to void formation. Higher helium production during irradiation appears to shorten the incubation period for void formation. The effects of helium on steady state void swelling behavior, however, remain unknown